duffro,
The '68 and early '69 750's had the points behind the cylinders and the tach drive off the cam thru the timing cover. For the '69 "S" model and all subsequent 750's they moved the points to the end of the cam and the tach drive to the front of the crankcase.
In 1972 they moved the crankcase breather from the end of the cam to the back of the crankcase.
A '73 engine in a '69 or '70 frame would be the different crankcase breather. (and of course the serial number).
Ron L